Men's Y chromosome is not about to go extinct

GOOD news for men: the Y chromosome will not mutate itself out of existence a few million years from now - confounding expectations.

Once upon a time, the Y sex chromosome looked much the same as the X sex chromosome. Both were X shaped, and matched up neatly. Like our other pairs of chromosomes, the two sex chromosomes exchanged genes as necessary to repair DNA and avoid harmful mutations.

Then something went badly wrong. Around 166 million years ago, a huge chunk of the Y chromosome in one of our mammalian ancestors was turned upside down and reinserted. The change was so extreme that the Y chromosome no longer matched the X, and it became impossible for the two to swap genes. The Y chromosome began collecting mutations and losing genes, ultimately taking on its characteristic Y shape as a result.

In humans, it now carries a mere 19 of the 800 genes it originally shared with the X. Given that rate of loss, some geneticists have predicted that the chromosome will lose its final gene in 4.6 million years.

Not so, says Jennifer Hughes at the Whitehead Institute for Biomedical Research in Cambridge, Massachusetts. She and her colleagues have just sequenced the Y chromosome of the rhesus macaque - a primate that diverged from humans around 25 million years ago.

They found that the monkey's Y chromosome contains 20 genes that match its X chromosome, and 19 of them are the same as human Y genes. This suggests that the human Y chromosome has lost only one gene since humans and macaques last shared a common ancestor (Nature, DOI: 10.1038/nature10843).

"We finally have empirical data that the Y chromosome has held steady over the last 25 million years," says Hughes. "Most of the Y chromosome's gene loss happened almost immediately after it stopped recombining with the X chromosome." The 19 surviving genes probably have vital biological functions, she says, and so aren't going anywhere anytime soon.

"It's a very nice piece of work," says Jenny Graves at the University of Melbourne, Australia, a proponent of the disappearing Y chromosome theory. However, Graves remains unconvinced that the Y will stick around for much longer. If versions of its 19 genes crop up on other chromosomes - which could happen at any time - those on the Y chromosome could be replaced, and males would no longer need the Y. They would become X0, because they would carry an X and an empty space where the Y used to be.

There is a precedent for this. Male Japanese Ryukyu spiny rats (Tokudaia osimensis) lack a Y chromosome. Darren Griffin, a geneticist at the University of Kent in Canterbury, UK, says male rats "carry on nicely" without it.

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Y chromosome is holding its own (Image: Department of Clinical Cytogenetics, Addenbrooke's Hospital/SPL/Getty)